Sunlight control glasses are commonly used in applications such as building glass windows and vehicle windows, typically offering high visible transmission and low emissivity. High visible transmission can allow more sunlight to pass through the glass windows, thus being desirable in many window applications. Low emissivity can block infrared (IR) radiation to reduce undesirable interior heating.
In low emissivity glasses, IR radiation is mostly reflected with minimum absorption and emission, thus reducing the heat transferring to and from the low emissivity surface. Low emissivity, or low-e, panels are often formed by depositing a reflective layer (e.g., silver) onto a substrate, such as glass. The overall quality of the reflective layer, such as with respect to texturing and crystallographic orientation, is important for achieving the desired performance, such as high visible light transmission and low emissivity (i.e., high heat reflection). In order to provide adhesion, as well as protection, several other layers are typically formed both under and over the reflective layer. The various layers typically include dielectric layers, such as silicon nitride, tin oxide, and zinc oxide, to provide a barrier between the stack and both the substrate and the environment, as well as to act as optical fillers and function as anti-reflective coating layers to improve the optical characteristics of the panel.
Low-emissivity coatings can also be engineered to provide desired shading properties. When sunlight reaches a window, a portion can pass through the window, a portion can be reflected back, and a portion cab be absorbed, which can heat up various parts of the window. A portion of the absorbed heat can flow to the inside of the house, heating up the air in the house. Thus, then sunlight hits a glass window, in addition to lighting the interior, the incident solar radiation can also pass through the window to warm up the house. Solar Heat Gain Coefficient (SHGC) is then defined as the fractional amount of the solar energy that strikes the window that ends up warming the house. Other terms can also be used, such as solar shading property or Light to Solar Gain (LSG), which is used to describe the relationship between lighting and heating from solar irradiation. Light to Solar Gain is defined as the ratio of visible light transmission to solar heat gain coefficient. In the hot weather, it is desirable to have high LSG glass. For example, commercial glass coatings are generally recommended to have LSG greater than 1.8.
There can be a tradeoff between having high visible transmittance and high light to solar gain. Transparent glass can provide high light transmittance but also high solar gain, e.g., low light to solar gain. Dark glass can provide low solar gain, but also low light transmittance. Ag coating low emissivity coating glass can provide significant improvements in terms of both visible light transmittance and light to solar gain properties. However, further improvements in light to solar gain is difficult, for example, low emissivity coatings having thicker Ag layer, or having multiple Ag layers, e.g., double Ag layer or triple Ag layer, can reduce the solar heat gain, but at the expense of lower light transmission.
Another desired characteristic of the glass coatings is color neutral property, e.g., colorless glass. The glass coatings should not exhibit observable hues, e.g., more red or blue than is desired.
Another desired characteristic of the glass coatings temperature matchability, e.g., similar performance and appearance before and after heat treatment. Since glass can be tempered, e.g., involving heating the glass to 600-700° C., the low-emissivity coatings can change significantly during the heat treatment process. To accommodate the tempering changes, low-emissivity coatings can be provided in a temperable version (e.g., heat treated) and a non-temperable version (non heat treated). The film stack of the temperable version can be designed to have properties matching those of the non-temperable version.
It would be desirable to provide low-emissivity coatings that can provide high visible transmittance, high light to solar gain, color neutral, and thermal stability for color and optical performance.